03-26-2012, 10:40 AM
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I am currently reading Ecology of the Planted Aquarium by D. Walstad and have come across something that might explain what you are currently experiencing.
Walstad argues that another cycle take place in the aquarium : denitrification. It's a process where bacteria, in anaerobic (no oxygen) conditions, take up nitrate, convert it to nitrite, then all the way through to pure nitrogen gas (there are quite a few steps). She points out that incomplete denitrification could cause a Nitrite spike under the right conditions.
Two factors seems to determine if denitrification is to take place. First, since nitrification (the cycle we all know about) is an oxygen-intensive activity : bacteria take up lots of oxygen to convert ammonia to nitrite and then to nitrate. This process can produce some oxygen-depleted (anaerobic) zones in the substrate or immerged filter that promotes denitrification. A second factor seems to be the type and depth of substrate : deep substrate and soil substrate promotes anaerobic bacterial growth, and thus the denitrification cycle.
My guess is that this may be happening in your aquarium. If that's the case, I would think that promoting more oxygenation in the substrate/filter would break the cycle. Also, feeding less may put less demand on oxygen. Walstad argues that the denitrification cycle is completely natural and will eventually balance itself in a planted aquarium. In the mean time, you will see elevated nitrites, wich is not too good for the fishes...